PSPad works under Win64 without any problem.
I didn't receive any report that installer 4.6.1 doesn't work under any Windows version

Download ZIP (portable version)
Unpack it anywhere you need and run.
If you untack it into program files folder, run PSPad as administrator then, open program settings / System integration and check [x]Multiuser environment option

In this case the problem is in your side.
Just tested and ZIP is OK.
Clear your browser cache, cause if you didn't download it correctly first time, your browser won't download it again, but offer it to you from cache.
There are 2 sources on the pspad.com web, one from FossHub, second is direct download.